1. Field of the Invention
This invention relates to materials testing apparatus, specifically, test cells for testing a specimen of material along three orthogonal axes.
2. Prior Art
For many materials such as rocks, salt, ice, concrete and the like it is important to measure a variety of properties under a triaxial state of stress, where the stresses act simultaneously along three orthogonal axes of the specimen. Such tests are conducted in a pressure vessel of the triaxial test cell that contains the specimen and a confining fluid and receives a piston to extend into that vessel. In such triaxial test cell, the vessel is typically narrow and can be open to admit the specimen and confining fluid. A piston is arranged to travel into the vessel from a cylindrical passage adjacent the test cell vessel. The piston compresses the fluid surrounding the specimen, applying the required triaxial hydrostatic stress thereto. In such arrangement, an axial loading force is applied to the loading piston that moves into the test cell vessel, pressurizing the confining fluid that applies a deviatoric axial stress to that test specimen, which stress is greater than a hydrostatic or reactive stress exerted by the pressurized fluid.
The above arrangement for applying the required deviatoric stress to the specimen has two drawbacks. Specifically, as the piston advances into the triaxial test cell, the pressurized fluid is further compressed, thus increasing the reactive fluid pressure when indeed it might be desirable to maintain confining pressure constant during a test; and, in order to apply the deviatoric stress to that test specimen, the actual force on the piston must be greater than the reactant force produced as the confining fluid acts on the cross sectional area of the piston face. The present invention, by providing a pressure balanced piston minimizes the magnitude of the external axial force required to generate a specified deviatoric stress in the test specimen.
Heretofore, to apply deviatoric stresses to a test specimen in a triaxial test cell the reactive force of the confining fluid had to be taken into account. The stepped piston and channeling arrangement of the present invention provides for a balancing where the force exerted by the confining fluid that is resistive to piston travel therein is cancelled.